Hydraulic brake system with hydraulic servo brake

ABSTRACT

A hydraulic brake system which incorporates several mutually independent brake circuits and does not require additional energy for conducting brake-slip control actions or stability control actions.

The present invention relates to a hydraulic brake system with a mastercylinder that can be actuated by means of a brake pedal, a brake lineconnecting the master cylinder with a brake cylinder, and a hydraulicunit with a motor-driven pump for boosting the brake force, wherein thedelivery flow of the pump is supplied to the suction side of the pumpvia a pressure control valve within a circuit.

Hydraulic brake systems of the type described above are known from theAte Brake Handbook, 2nd edition, 1960, pp. 60-65. In the brake systemsdescribed in the handbook, a motor-driven high-pressure pump delivers aconstant flow of fluid that circulates within a circuit, flowing througha restrictor gap between a boosting valve located on the master cylinderand the piston of the master cylinder and then back to a reservoir, fromwhich the pump draws in the pressure fluid. When the brake pedal isactuated, the boosting valve is displaced against the piston of themaster cylinder, and this causes the flow in the restrictor gap to berestricted. This in turn causes pressure to be built up in thering-shaped space between the boosting valve and the piston of themaster cylinder. This pressure displaces the piston of the mastercylinder in the direction of braking, and it becomes effective as pedalreturn force at the boosting valve. Due to the comparably highermanufacturing and operating costs as compared to vacuum brake forceboosters, these known brake systems with hydraulic brake force boostingnever achieved significance in practice.

Another known device for hydraulically boosting the brake force uses theenergy supply already available in the motor vehicle by having thehydraulic pump for the steering servo charge a hydraulic accumulator viaa pressure-controlled current regulator. The pressure fluid stored underpressure is led to a boosting piston via a control valve that isactuated by the brake pedal, and this boosting piston actuates thepiston of a master cylinder. In this design, an additional pump circuitis required for a brake-slip control.

In another hydraulic brake system, known from DE 40 35 906 Al, thepressure fluid delivered by a pump is supplied, via a pressure reducingvalve, to a pressure control valve that is actuated by the brake pedal.The pressure control valve regulates the pressure in the brake linedepending on the force exerted on the brake pedal. The pressure reducingvalve is connected to the brake line via a control line and arranged insuch a way that the pressure on the inlet side of the pressure reducingvalve always is 30 bar higher than the pressure in the brake line. Thisknown brake pressure control device is not suitable for connectingseveral mutually independent brake circuits.

DE 44 46 525 Al describes a hydraulic motor vehicle brake system fordriving stability control and anti-slip control with a brake-slipcontrol system that works on the basis of the return principle, whereinthe dual-circuit master cylinder is actuated by a vacuum brake forcebooster that can be regulated by means of a brake pedal. The brakesystem has a pump whose pressure side is connected to the brake line,and said pump operates as a recirculating pump for brake-slip controland as a brake pressure transducer for driving stability or anti-slipcontrol. The pump is not in operation during braking procedures that donot use the control devices. When the pump operates as a brake pressuretransducer, the suction side of the pump can be connected to the brakeline via a valve, with a precharging pressure being generated in thebrake line by means of a precharging pump in order to improve the intakeaction.

The object of the present invention is to create a brake system of thetype mentioned above, which, firstly, is characterized by a simpledesign and low construction costs, secondly, can have several, mutuallyindependent brake circuits, and, thirdly, does not need an additionalenergy supply for brake-slip control actions or an automatic brakeactuation, e.g. driving stability control.

According to the present invention, this object is achieved in that thepump, the pressure control valve and a non-return valve that closes inthe direction of the master cylinder are arranged parallel to oneanother in the brake line of a brake system of the type mentioned above,wherein the suction side of the pump and the outlet of the pressurecontrol valve are connected to the master cylinder and the pressure sideof the pump and the inlet of the pressure control valve are connected tothe brake cylinder and wherein the pressure control valve regulates thepump pressure in dependence of the pressure in the section of the brakeline connected to the master cylinder and the hydraulic surfaces of thepressure control valve are designed in such a way that the ratio betweenpump pressure and master cylinder pressure is greater than 1.

In the brake system according to the present invention the actuatingforce exerted on the brake pedal is not boosted in the master cylinderor brake pressure transducer, but in the brake line, wherein theintegrated hydrodynamic pump circuit divides said brake line into asection with low pressure on the side of the master cylinder and asection with higher pressure on the side of the brake cylinders. Thepump circuit also is not controlled mechanically by means of the brakepedal but through the output pressure of the master cylinder, which isapplied to the control piston of the pressure control valve. Thehydrodynamic pump circuit is closed with the exception of theattachments for the brake line and, consequently, it also is part of thehydrostatic brake circuit. Thus, whenever the brake force is boosted,the flow quantity of the pump always corresponds only to the pressuredifference between master cylinder pressure and brake cylinder pressure.The non-return valve arranged parallel to the pump circuit ensures adirect connection between the master cylinder and the brake wheelcylinder in order to obtain a quick brake actuation independently of thepump circuit. When the pressure in the master cylinder is reduced, thepressure in the brake cylinder is reduced through the pressure controlvalve.

The brake system according to the present invention allows hydraulicbrake force boosting with low structural costs and irrespective of otherservo-systems in the vehicle. It is particularly well-suited for motorvehicles that do not have available an adequate vacuum for operating apneumatic vacuum brake force booster. Since the brake force boosting iscontrolled only hydraulically in the brake system according to thepresent invention, it can be used with any type of master cylinder. Italso is possible to integrate a hydrodynamic boosting circuit accordingto the present invention in an already existing brake system.Furthermore, the brake system according to the present invention offersthe advantage that a brake-slip control can be obtained through fewadditional measures and devices, because the existing pump is used as arecirculating pump. In the same way the brake system according to thepresent invention can be expanded into a brake system with drivingstability control through few additional measures and devices.

According to the present invention, an electric motor can be used todrive the pump, with said electric motor being switched on when abraking procedure is initiated. This does not detrimentally affect theresponse behavior of the brake system, since the pressure build-up whilethe pump is starting up is supported by the master cylinder.

According to the present invention, the pressure control valve can bedesigned as a pressure limiting valve, for which purpose a non-returnvalve closing in the direction of the pressure limiting valve isarranged between the outlet of the pressure limiting valve and the pointwhere the control line of the pressure limiting valve runs into thebrake line. Furthermore, it may be provided that the control piston ofthe pressure control valve be designed as a stepped piston withatmospheric pressure being applied to its stepped surface.

A further embodiment of the brake system according to the presentinvention, which allows control of the brake slip, may be achieved inaccordance with the present invention in that the suction side of thepump can be disconnected from the master cylinder, from the non-returnvalve located in the brake line and from the pressure control valve bymeans of a block valve, in that the supply of pressure fluid to thebrake cylinder can be blocked by a first control valve and the brakecylinder can be connected to a return line leading to a low-pressureaccumulator and to the suction side of the pump by means of a secondcontrol valve, and in that the block valve and the control valves can becontrolled by a brake-slip control device.

By adding just a few components to such a brake system, an automaticbrake actuation to control the driving stability can be realized.According to the present invention, such expansion may consist ofequipping the master cylinder with a device for precharging the brakesystem and providing a stop valve with a parallel pressure limitingvalve in line with the pressure control valve, wherein the prechargingdevice, stop valve, block valve and control valves can be regulated bymeans of a driving stability control device. An electromechanicalprecharging drive is particularly suitable for use as a prechargingdevice. It is provided on the master cylinder and is equipped with anelectric motor and gearing that transforms the rotary motion of theelectric motor into a linear motion to actuate the piston of the mastercylinder. If the master cylinder is not to be used for the prechargingprocedure, then, according to the present invention, a charge pumpdriven by an electric motor may be provided. Said charge pump isconnected to the reservoir attachment of the master cylinder, whereinthe port between the reservoir attachment and the master cylinder can beblocked by a valve. When the master cylinder is a tandem mastercylinder, it would suffice to connect the charge pump to the reservoirattachment of the plunger rod circuit. The floating circuit is thenprecharged by hydraulically displacing the floating piston of the tandemmaster cylinder.

Since the charge pump has to deliver only a limited volume of pressurefluid to the brake system, it may, according to the present invention,consist of a charge cylinder resembling a master cylinder whose pistonis displaced by an electromechanical drive. The electromechanical drivemay be equipped with a sliding clutch in order to limit the pressuregenerated by the precharging procedure.

The present invention is described in more detail on the basis ofembodiments shown in the following drawings:

FIG. 1 is the circuit diagram of a brake circuit of a hydraulicdual-circuit brake system with hydraulic brake force boosting accordingto the present invention.

FIG. 2 is the circuit diagram of a brake circuit of a brake systemaccording to FIG. 1, which was expanded to comprise a brake-slip controldevice by adding the corresponding components.

FIG. 3 is the circuit diagram of a brake circuit of a brake systemaccording to FIG. 2, which was expanded to comprise a driving stabilityand traction control device by adding the corresponding components.

FIG. 4 shows an axial section of an embodiment of a pressure controlvalve.

FIG. 5 is a schematic representation of an electromechanically actuatedcharge cylinder.

FIG. 6 shows an axial section of a master cylinder with anelectromechanical precharging drive.

FIG. 1 shows one of two identical brake circuits that are connected tothe two working chambers of a tandem master cylinder 1 with a reservoir2 for pressure fluid. A brake pedal 3 is provided, by means of which thetandem master cylinder 1 is actuated. The brake circuit has a brake line4 that is made up of a section 5 on the side of the master cylinder andsections 8 which are connected to two brake cylinders 6, 7. The sections5, 8 are connected by the lines 9, 10 of a pump circuit and line 11which is provided with a non-return valve 12 that closes in thedirection of the tandem master cylinder 1. A pump 13 with intake valve14 and pressure valve 15 is arranged in line 9, wherein said pumpdelivers, via a damping chamber 16 and a throttle 17, in the directionof the sections 8 of the brake line 4. The pump 13 is driven by anelectric motor 18. A pressure limiting valve designed as a pressurecontrol valve 19 which opens in the direction of the tandem mastercylinder 1 is provided in line 10. A non-return valve 20 is provideddownstream of the pressure control valve 19. The pressure control valve19 is connected to the line 10 via a control line 21 parallel to thenon-return valve 20. Another control line 22 connects the pressurecontrol valve 19 with the section of line 10 on the side of the brakecylinder.

When the tandem master cylinder 1 is actuated by the brake pedal 3, thebrake fluid displaced as a result is supplied, via the line 11 and thenon-return valve 12, to the brake cylinders 6, 7 so that the brakes arequickly filled and applied. At the same time the electric motor 18 isswitched on, e.g. by means of a switch actuated by the brake pedal,causing the pump 13 to start up. Since the pressure control valve 19 isheld closed by the pressure built up in the meantime by the tandemmaster cylinder, the flow of fluid delivered by the pump 13 as it startsup causes an increase in pressure that exceeds the master cylinderpressure in the sections 8 of the brake line and the brake cylinders 6,7. The pressure increase is limited by the pressure control valve 19,which will not open until the proportional ratio between brake cylinderpressure and master cylinder pressure as it is determined by the designof the hydraulically effective surfaces of the pressure control valve 19is reached. This pressure ratio is maintained by the pressure controlvalve for the duration of the brake actuating procedure; however, thepressure control valve restricts the flow of the pump 13 more or lessstrongly depending on the master cylinder pressure in order to adjustthe amount of the brake cylinder pressure accordingly. When the mastercylinder pressure is reduced by releasing the brake pedal 3, i.e. to endthe braking procedure, the brake fluid flows from the brake cylinders 6,7 back to the tandem master cylinder 1 via the open pressure controlvalve 19. The pump 13 is switched off as soon as the brake pedal 3reaches its original position or the pressure in the master brakecylinder 1 is reduced completely.

In the further embodiment of the brake circuit described above and shownin FIG. 2, an electromagnetically actuatable intake valve 23 that isopen in its inactive position and can be closed by exciting theactuating magnet is integrated in each section 8 of the brake line 4leading to a brake cylinder 6 or 7. Non-return valves 24 opening in thedirection of the pump circuit are provided parallel to the intake valves23. In addition, each of the brake cylinders 6, 7 is connected to areturn line 26 via an electromagnetically actuatable discharge valve 25that is closed in its inactive position and can be opened by excitingthe actuating magnet. The return line 26 leads to a low-pressureaccumulator 27 and to the suction valve 14 of the pump 13 via anon-return valve 28. Furthermore, an electromagnetically actuatableblock valve 29, which is open in its inactive position, is provided inthe line 9 between the attachments of the return line 26 and the line11. The line 9 and, hence, the connection between the suction side ofthe pump 13 and the tandem master cylinder 1 and the pressure controlvalve 19 can be blocked with said block valve 29.

The magnets of the intake valves 23, the discharge valves 25 and theblock valve 29 are controlled by an electronic brake-slip control unitwhich emits valve control signals to modulate the brake pressure in thebrake cylinders 6, 7 when control operations are required. If, forexample, the brake pressure in the brake cylinder 6 is to be reduced,the intake valve 23 of the brake cylinder 6 is closed and the dischargevalve 25 is opened. As a result, pressure fluid flows from the brakecylinder into the return line 26 via the discharge valve 25, and then itflows into the low-pressure accumulator 27. At the same time, the blockvalve 29 is closed, whereupon the pressure on the suction side of thepump 13 drops and the pump 13 is forced to return the pressure fluidvolume contained in the low-pressure accumulator 27 to sections 8 of thebrake line 4 or, if it cannot be contained there, to return it to thetandem master cylinder 1 via the pressure control valve 19. In order tobuild up pressure again, the intake valve 23 and the discharge 25 of thebrake cylinder 6 are reset to their inactive positions. If the pressurefluid volume available in the low-pressure accumulator does not sufficefor building up the required pressure, the block valve 29 is openedagain, so that the pump 13 can remove the required pressure fluid fromthe tandem master cylinder 1. When the control procedure is over, theblock valve 29 remains blocked until the pump has emptied out thelow-pressure accumulator 27 and delivered the pressure fluid volume backto the tandem master cylinder 1.

An expansion of the brake system according to FIG. 2 is shown in FIG. 3.In this expanded system an automatic, electronically controlled brakeactuation to control the driving stability or the traction slip ispossible. For this purpose, an electromagnetically actuatable stop valve30 is additionally provided in the line 10 downstream of the attachmentof the control line 21. A non-return valve 31 opening in the directionof the control line 21 and a pressure limiting valve 32 opening in thedirection of the brake line 4 are arranged parallel to said stop valve30. Furthermore, the tandem master cylinder 1 is provided with a devicefor precharging the brake circuits, with such device consisting of anelectromechanically driven charge cylinder 33 and a solenoid valve 34.The charge cylinder 33 is described in more detail below in connectionwith FIG. 5. It is connected to the reservoir 2 via a low-pressure line35 and to the reservoir attachment 37 of the working chamber of thetandem master cylinder 1 on the side of the plunger rod via a pressureline 36. The port between the reservoir 2 and the reservoir attachment37 can be blocked by the solenoid valve 34.

When an automatic brake actuation is to be initiated, the control unitswitches on the drive of the charge cylinder 33 and the electric motor18 of the pump 13 and, at the same time, closes the stop valve 30 andthe solenoid valve 34. The charge cylinder 33 fills the brake circuitand a precharge pressure of up to 6 bar is built up, which ensures thatbrake pressure will be generated quickly with the aid. of the pump 13.The precharge pressure build-up in the plunger rod circuit of the tandemmaster cylinder 1 also displaces the floating piston of the tandemmaster cylinder 1; moreover, this causes build-up of a correspondingprecharge pressure in the second brake circuit (not shown) connected tobrake line 38, the design of which corresponds to that of the brakecircuit shown. By closing the stop valve 30, the entire pressure fluidvolume delivered by the pump 13 is available for building up pressure inthe sections 8 of the brake line 4. As in the brake-slip control, thepressure fluid volume can be supplied to the brake cylinders 6, 7 for acontrolled pressure modulation by regulating the intake valves 23 andthe discharge valves 25 as well as the block valve 29 appropriately. Theexcessive pressure fluid volume is returned to the tandem mastercylinder 1 or the charge cylinder 33 via the pressure control valve 19and the pressure limiting valve 32. For this purpose, the openingpressure of the pressure limiting valve 32 and the switching pressure ofthe pressure control valve 19 have to be coordinated with one another insuch a way that an appropriate maximum pressure can be built up in thesections 8 of the brake line 4.

FIG. 4 shows an embodiment of the pressure control valve 19. A steppedpiston 41 is displaceably mounted in a cylinder housing 39 with astepped cylinder bore 40 and sealed with sealing rings 42, 43. With itssmaller face side the stepped piston 41 borders on a valve housing 44and with its larger face side it borders on a control housing 45. Thevalve housing 44 holds a valve ball 46 and a pressure spring that holdsopen the valve ball when it is in an unpressurized state; when activethe valve ball closes a valve seat 47. A bore 48 connects the valve seatto the line 4 leading to the pressure side of the pump 13. A bore 49connects the valve housing 44 to the section of line 10 that is attachedto the suction side of the pump 13. The control housing 45 is connectedto the control line 21 via a bore 50. The ring-shaped space 51 delimitedby the piston step is connected to the atmosphere by means of a bore 52.

The drawing shows the pressure control valve in a closed state, whichoccurs when pressure is applied to the control line 21. The closingforce is determined by the size of the ring-shaped surface on the stepof the stepped piston 41. The pressure control valve can only be openedwhen the pressure in the bore 48 exceeds the pressure in the valvehousing 44 and the control housing 45 by a quantity that corresponds tothe pressure in these housings multiplied by the quotient of thering-shaped surface of the piston step and the cross-sectional area ofthe valve seat 47. Thus, the closing force increases proportionally tothe pressure built up in the valve housing 44 and control housing 45 bythe master cylinder, with the ratio between closing force and mastercylinder pressure being determined by the relation between theabove-mentioned surfaces. When the brake is released and the mastercylinder pressure is reduced to zero, the valve is opened by the fluidflowing back to the master cylinder as well as a pressure spring 77, andit remains open until pressure is built up again.

FIG. 5 shows an embodiment of the electromechanically actuatable chargecylinder 33. The design of the charge cylinder 33 corresponds to that ofa master cylinder with a cylinder housing 53, a piston 54 and a centralvalve 55 mounted in a piston 54. A pressure spring 56 pushes the piston54 to the inactive position shown in the drawing, in which the centralvalve 55 is held open by a pin 57. In this position the central valve 55connects a pressure chamber 58 delimited by the face side of the piston54 with a reservoir chamber 60 that can be connected to a reservoir viaa bore 59. Attached to the piston 54 is a gear rack 61 which is in meshwith a gear wheel 62 of a gearing. The gear wheel 62 is connected to aworm gear 64 by means of a sliding clutch 63, with the worm gear 64being engaged in a worm 66 that is driven by an electric motor 65. Anyother type of reducing gear can be used in the place of a worm gearing.The sliding clutch 63 in the worm gear 64 is designed in such a way thatit slides through when there is a precharge pressure of 6 bar in thepressure chamber 58 and accordingly also when the piston 54 reaches itsend position.

When the electric motor 65 is switched on to activate the prechargeprocedure, it drives the sliding clutch 63 via the worm gearing 66, 64and the rack-and-pinion gear 61, 62 drives the piston 54, which causespressure fluid to be delivered to the reservoir attachment 37 of thetandem master cylinder 1 and through this to the brake circuit. When theprecharge pressure reaches 6 bar, the sliding clutch prevents anyfurther increase in pressure. Thus, how long the electric motor 65remains switched on can be determined by the largest possible prechargestroke. Pressure-controlled regulation is not required. When the pump 13returns excess volume to the charge cylinder 33, this pushes the piston54 back without the electric motor 65 being switched on, because in thiscase the sliding clutch 63 can slip through, too. Consequently, theelectronic control is rendered particularly simple.

As the embodiment described in FIG. 6 shows, the precharging requiredfor automatic braking procedures can also be achieved by means of anelectromechanically actuatable tandem master cylinder 67 instead of acharge cylinder. For this purpose, the open end of the tandem mastercylinder housing 68 is fitted with an extension 69 with a gear racksleeve 70 which supports the plunger rod piston 71. A plunger 72 that isattached to the plunger rod piston 71 is located within the bore of thegear rack sleeve 70, and the plunger rod 73 is attached to said plunger72 by means of a socket joint. A gear wheel 74 which displaces the gearrack sleeve 70 is arranged in a tangential bore of the extension 69. Thegear wheel 74 is driven in the same way as described in the embodimentaccording to FIG. 5, i.e. by means of an electric motor 76 and a gearing75.

When the tandem master cylinder 67 is actuated in the normal way, i.e.by the brake pedal, the plunger 72 glides in the gear rack sleeve 70,without the feeling on the pedal being affected. When the prechargedrive is switched on, the plunger rod piston 71 is displaced by the gearrack sleeve 70 in the direction in which the brakes are applied. Hereagain a sliding clutch is active in the gearing 75, which limits theactuation forces in both directions.

LIST OF REFERENCE NUMBERS

1 Tandem master cylinder

2 Reservoir

3 Brake pedal

4 Brake line

5 Section

6 Brake cylinder

7 Brake cylinder

8 Section

9 Line

10 Line

11 Line

12 Non-return valve

13 Pump

14 Suction valve

15 Pressure valve

16 Damping chamber

17 Throttle

18 Electric motor

19 Pressure control valve

20 Non-return valve

21 Control line

22 Control line

23 Intake valve

24 Non-return valve

25 Discharge valve

26 Return line

27 Low-pressure accumulator

28 Non-return valve

29 Block valve

30 Stop valve

31 Non-return valve

32 Pressure limiting valve

33 Charge cylinder

34 Solenoid valve

35 Low-pressure line

36 Pressure line

37 Reservoir attachment

38 Brake line

39 Cylinder housing

40 Cylinder bore

41 Stepped piston

42 Sealing rings

43 Sealing rings

44 Valve housing

45 Control housing

46 Valve ball

47 Valve seat

48 Bore

49 Bore

50 Bore

51 Ring-shaped space

52 Bore

53 Cylinder housing

54 Piston

55 Central valve

56 Pressure spring

57 Pin

58 Pressure chamber

59 Bore

60 Reservoir

61 Gear rack

62 Gear wheel

63 Sliding clutch

64 Worm gear

65 Electric motor

66 Worm

67 Tandem master cylinder

68 Master cylinder housing

69 Extension

70 Gear rack sleeve

71 Plunger rod piston

72 Plunger

73 Plunger rod

74 Gear wheel

75 Gearing

76 Electric motor

What is claimed is:
 1. A hydraulic brake system, comprising: a mastercylinder that can be actuated by a brake pedal, a brake line connectingthe master cylinder to a brake cylinder, and a hydraulic unit forboosting the brake force with a pump driven by a motor, wherein thepumps delivery flow is supplied to a suction side of the pump by way ofa pressure control valve within a circuit, wherein the pump and thepressure control valve are arranged parallel to one another in the brakeline, wherein the suction side of the pump and an outlet of the pressurecontrol valve are connected to the master cylinder and the pressure sideof the pump and an inlet of the pressure control valve are connected tothe brake cylinder, and wherein the pressure control valve regulates thepump pressure in dependence of the section of the brake line that isconnected to the master cylinder and a hydraulically effective surfaceof the pressure control valve are designed in such a way that the ratiobetween pump pressure and master cylinder pressure is greater than 1, anon-return valve closing in the direction of the master cylinder isarranged parallel to the pump and to the pressure control valve in thebrake line; a non-return valve that closes in the direction of thepressure control valve is arranged at the outlet of the pressure controlvalve which runs in the direction of a line connected to the section onthe side of the master cylinder; a restrictor which interconnects thepump and the brake cylinder, wherein, the suction side of the pump isdisconnected from the master cylinder by a block valve, from thenon-return valve arranged in the brake line and from the pressurecontrol valve, in that a supply of pressure fluid to the brake cylindercan be blocked by a first control valve arranged in the brake line andthe brake cylinder can be connected to a return line leading to alow-pressure accumulator and to the suction side of the pump by means ofa second control valve, and in that the block valve and the controlvalves can be controlled by a brake-slip control unit, wherein themaster cylinder is connected to a unit for precharging the brake system,and that a stop valve with a parallel pressure limiting valve isarranged in line with the pressure control valve, wherein theprecharging unit, the stop valve, the block valve and the control valvescan be regulated by a driving stability or anti-slip control unit,wherein the device for precharging the brake system is a charge pumpdriven by an electric motor, which is connected to the reservoirattachment of the master cylinder, wherein the port between thereservoir attachment and the reservoir is switched by a valve.
 2. Abrake system according to claim 1, wherein the pump is driven by anelectric motor, wherein said electric motor is switched on as soon as abraking procedure is initiated.
 3. A brake system according to claim 1,wherein the pressure control valve is designed as a pressure limitingvalve, and that a control line which is connected to the pressurecontrol valve is arranged between the non-return valve and the sectionof line that is on the side of the master cylinder.
 4. A brake systemaccording to claim 3, the pressure control valve includes a controlpiston designed as a stepped piston, with atmospheric pressure beingapplied to its stepped surfaces and discharge pressure being applied toits end faces.
 5. A brake system according to claim 1, wherein thecharge pump consists of a charge cylinder, having a piston, wherein thepiston can be displaced by an electromechanical drive.
 6. A brake systemaccording to claim 1, wherein, the unit for precharging the brake systemis an electromechanical drive that is provided on the master cylinder,with said drive displacing the piston of a master cylinder.
 7. A brakesystem according to claim 1, further including a sliding clutch attachedbetween the charge pump and the electric motor.
 8. A brake systemaccording to claim 5, wherein the electromechanical drive has anelectric motor and a gearing which transforms the rotational motion ofthe electric motor into a straight-line motion.